1. Field of the Invention
The invention relates in general to a slant reflector with a bump structure and a fabricating method thereof, and more particularly to the slant reflector with bump structure fabricated by one photo-mask and applied in the reflective type liquid crystal display (LCD).
2. Description of the Related Art
In recent years, it is not only the brightness but also the viewing angle required for the reflective type liquid crystal display (LCD) that has become important in the commercial market. It is a very important issue for the manufacturers and researchers to develop a reflective type LCD with a high brightness and a wide viewing angle.
FIG. 1A is a light path of a horizontal reflector with a smooth surface. The dash line represents a normal (perpendicular) to the surface of the reflector 100. The reflector 100 is set up horizontally, and the surface thereof is smooth. It is assumed that an angle of incidence of the incident light arriving at the surface of reflector is 20 degree. According to the optical theory, the angle of reflectance of the reflected light is −20 degree. Hence, the maximum reflectance R1 occurs at the viewing angle of −20 degree, and the curve of reflectance distribution is very narrow, mostly in the region around −20 degree. FIG. 1B represents the reflectance distribution of the conventional reflector of FIG. 1A. The reflectance is measured by an optical detection system.
However, the ideal LCD represents the maximum reflectance at the viewing angle of 0 degree, and distributes parts of reflectance at more wide range of the viewing angle. To shift the curve of FIG. 1 towards the left, it is another conventional method which slants the reflector to change the light path. FIG. 2A is a light path of a slant reflector with a smooth surface. The dash line represents a normal (perpendicular) to the upper surface 200′ of the reflector 200. The reflector 200 is set up with an inclined angle (10 degree), and the upper surface 200′ thereof is smooth. Since the angle between the upper surface 200′ and the horizontal plane is 10 degree, the original incident light at 20 degree of incidence can be reflected at the angle of 0 degree, as demonstrated in FIG. 2A. FIG. 2B represents the reflectance distribution of the conventional reflector of FIG. 2A. The maximum reflectance R1 occurs at the viewing angle of 0 degree, but the curve of reflectance distribution is still narrow. The results of reflectance mostly occur in the region around 0 degree. The objective of the ideal reflective type LCD with wide viewing angle has not been achieved.
Another conventional method is further provided to form numerous bumps on the slant reflector. FIG. 2C is a light path of another slant reflector with a smooth surface. FIG. 2C provides a curving surface having an angle of 10 degree such that numerous normals perpendicular to the curving surface of the reflector 300 are generated. Accordingly, the angles of reflection generated by the reflector 300 of FIG. 2C are more than the reflector 200 of FIG. 2A. However, the curve of reflectance distribution is still narrow, and the reflectance mostly occurs in the region around 0 degree. The reflectance distribution of the reflector 300 of FIG. 2C is similar to FIG. 2B.
FIG. 3 represents the ideal reflectance distribution of the reflector. The reflector is set up with an inclined angle (10 degree), and there are numerous bumps formed on the slant surface of the reflector. The high reflectance is detected around the angle of 0 degree due to the slanted reflector. Also, part of reflectance is detected over a wide range of viewing angles since the normals on each point of the bump are not parallel. Comparing the results of FIG. 2B and FIG. 3, the maximum reflectance R2 of FIG. 3 is lower than the maximum reflectance R1 of FIG. 2, but the reflectance distribution of FIG. 3 is wider than that of FIG. 2. Therefore, the LCD adopting the slant reflector with bump structure possesses two attractive features—high brightness and wide viewing angle.
FIG. 4A˜FIG. 4F show a conventional method of fabricating a slant reflector with a bump structure. In this conventional process, a photo-mask 406 with a single slit is provided, and multi-step exposure is performed. First, the photosensitivity material such as photo-resist 404 is deposited on the substrate 402. Then, the photo-mask 406 with the single slit is utilized for the pattern transfer. It is assumed that the photo-resist 404 is a positive photo-resist dissolving in the developer. In FIG. 4A, the photo-resist 404 is exposed to the UV (Ultraviolet) light at an intensity of L1 for a time t1, and an exposed area A is formed. Second, the photo-mask 406 is shifted to the right, and the photo-resist 404 is exposed to the UV (Ultraviolet) light at an intensity of L2 for a time t2 to form an exposed area B, as shown in FIG. 4B. Next, the photo-mask 406 is shifted to the right again, and the photo-resist 404 is exposed to the UV (Ultraviolet) light at an intensity of L3 for a time t3 to form an exposed area C, as shown in FIG. 4C. Either by setting equal exposing times and the light intensity at L1>L2>L3, or by setting equal intensities and the exposing time at t1>t2>t3, the sizes of the exposing areas are consequently controlled at the order of A>B>C.
Subsequently, the development is performed on the photo-resist 404 to form a ladder-like appearance, as shown in FIG. 4D. Then, the ladder-like photo-resist 404′ is melted by heat treatment. When the heating temperature is increased up to the glass transition temperature of photo-resist 404, the photo-resist 404′ will be softened as the melting glass and reflowed to form a slant photo-resist 404″ with a smooth surface, as shown in FIG. 4E. Next, a second photo-mask (not shown) is provided to form a plurality of bumps 408 on the slant photo-resist 404″, as shown in FIG. 4F. Finally, a metal film (not shown) is deposited on the slant photo-resist 404″, and also covering the bumps 408.
The ideal reflectance distribution (FIG. 3) can be achieved by the slant reflector with a bump structure, fabricated by the conventional process (FIG. 4A˜FIG. 4F). However, this conventional process has several drawbacks. For example: the photo-mask needs to be shifted over and over again, and the position of the photo-mask, UV light intensity or duration time needs to be adjusted while the photo-mask is shifted. It is time-consuming, and the production cost is consequently raised. It is required to shift the photo-mask and expose the photo-resist at least for three times, in order to form a slant photo-resist (404″). If numerous slant photo-resists are required to create the tilt surface of the reflector for enhancing the light scattering effect, the conventional process for fabricating thereof is time-consuming and not suitable for the mass-production scale. Also, the conventional process requires at least two photo-masks. One is used for fabricating the slant surface, and the other for forming a plurality of bumps on the slant surface. It is very inconvenient.